1. Field of the Invention
The present invention relates to an electrophoresis device using an electrophoresis phenomenon, a method of driving the electrophoresis device, and an electronic apparatus including the electrophoresis device.
Priority is claimed on Japanese Patent Application No. 2005-040229, filed Feb. 17, 2005, the image of which is incorporated herein by reference.
2. Description of Related Art
As an electrophoresis phenomenon, a phenomenon in which charged particles dispersed in a liquid are migrated by an electric field has been generally known. As a technology for applying this phenomenon, there has been known a technology that, when an electric field is applied between a pair of electrodes in a state in which one formed by dispersing charged pigment micro particles into a dispersion colored with a dye is inserted between the pair of electrodes, the charged particles are attracted by any one of the electrodes. Efforts for implementing a display device by using the phenomenon have conventionally been made. A material formed by dispersing the charged particles into the dispersion colored with the dye is called electro phoretic ink, and a display device using the electro phoretic ink is called an electro phoretic display (EPD).
When the electric field is applied to the electro phoretic ink from the outside, the charged particles move in a direction of the electric field in the case where the charged particles are charged with the positive polarity, and the charged particles move in a direction opposite to the direction of the electric field in the case where the charged particles are charged with a negative polarity. As a result, the side from which the electro phoretic ink is seen, that is, a display surface is seen like being colored with any of the color of a solvent and the color of the charged particle. Therefore, the movement of the charged particles of the electro phoretic ink that is located on each pixel surface is controlled for every pixel, so that display information can be displayed on the display surface.
In recent years, there has been suggested a technology that the electro phoretic ink is filled into the microcapsule to constitute the electro phoretic ink in a microcapsule manner, thereby improving the reliability of display. Two kinds of charged particles composed of a charged particle having a color forming display and a charged particle having a color forming a background are filled into the microcapsule. In other words, the electro phoretic ink made in the microcapsule manner is coated on an active matrix type of element array to achieve a display device (electrophoresis device) with excellent visibility and low power consumption.
However, the electrophoresis device formed by combining the electro phoretic ink constructed in the microcapsule manner and the active matrix type of element array has problems of a driving method as follows.
The voltage (a difference in electric potential) required when the displayed image is changed depends on the size of the microcapsule (diameter) and has approximately 1 V/μm. The diameter of a general microcapsule is several tens μm, so that the voltage needs at least 10 V. Here, it is described the case in which the driving voltage is set to 10 V and a typical method of driving a liquid crystal display is applied to the electrophoresis device.
First, the voltage applied to the common electrode is set to 10 V, and the voltage applied to the pixel electrode is set to 0 V or 20 V. In other words, when the electric potential of the common electrode is greater than that of the pixel electrode, the voltage applied to the pixel electrode is set to 0 V. To the contrary, when the electric potential of the pixel electrode is greater than that of the common electrode, the voltage applied to the pixel electrode is set to 20 V. Therefore, the displayed image can be rewritten.
However, for the voltage applied to the pixel electrode, the driving voltage is too high when switching the TFT connected to the pixel electrode, so that it is difficult to obtain the reliability of the TFT. In addition, a voltage of 20 V is only an approximate value, and the voltage may be 30 V or more. In this case, it is further difficult to obtain reliability.
In addition, as another typical method of driving the liquid crystal display, a method that the electric potential of the common electrode is changed is known, which is called a common swing method. In other words, when the electric potential of the common electrode is greater than that of the pixel electrode, the voltage applied to the pixel electrode is set to 0 V, and the voltage applied to the common electrode is set to 10 V. To the contrary, when the electric potential of the pixel electrode is greater than that of the common electrode, the voltage applied to the pixel electrode is set to 10 V, and the voltage applied to the common electrode is set to 0 V. As a result, the displayed image can be rewritten at a voltage of 10 V, and the reliability of the TFT can be improved.
However, this method has the following problems.
For example, it is assumed that the voltages of 10 V and 0 V are respectively applied to the common electrode and the pixel electrode in order to rewrite the displayed image of any pixel. In this case, the voltage of 10 V must be applied to the other pixel electrodes to which the displayed image is not rewritten, in order to prevent an erroneous rewriting operation. However, since applying the voltage to each pixel electrode is performed by sequentially selecting each pixel transistor, the timing when applying the voltage to each pixel electrode does not coincide with the timing when applying the voltage to the common electrode, so that delay occurs. As a result, there is a fear that the erroneous rewriting occurs. In addition, even though the voltage is applied to each pixel electrode before the erroneous rewriting occurs, the voltage of the pixel electrode gradually decreases due to the leakage of the pixel transistor. There is a possibility that the erroneous rewriting will occur.
Therefore, as a conventional art for solving these problems, there is provided a display device (electrophoresis device) in which, when the displayed image is changed, the image displayed by that time is deleted over the entire display region and new display image is written on the display region (for example, see Japanese Unexamined Patent Application Publication No. 2002-149115).
In other words, all the plurality of pixel electrodes is set to have the same electric potential, the voltage is applied between the common electrode and the pixel electrode, and the image displayed by that time is deleted over the entire display region. After that, when the new display image is rewritten on the display region, the electric potential of the common electrode is the same as that of the pixel electrode, and a predetermined electric potential is applied to the pixel electrode to be rewritten.
By driving in this manner, it is possible to prevent erroneous rewriting as described above.
However, the above-mentioned conventional display device (electrophoresis device) has the following problems.
FIGS. 13A and 13B are diagrams for illustrating the problems of the display device, where reference numeral 1 indicates a plurality of pixel electrodes provided on a first substrate (not shown) and reference numeral 2 indicates a common electrode provided on a second substrate (not shown). A liquid material (not shown) containing black particles 3 and white particles 4 is sealed between the pixel electrodes 1 and the common electrode 2 so as to be interposed therebetween. The black particles 3 are colored with black, functioning as a display color, and are charged with a positive polarity, and the white particles 4 are colored with white, functioning as a background color, and are charged with a negative polarity. In the display device (electrophoresis device), the common electrode 2 forms the display surface. In addition, the liquid material is commonly used with the microcapsule type. However, in this case, the description of the microcapsule is omitted for the simplicity of description.
In the above-mentioned display device, when the displayed image is changed, the image displayed by that time is deleted over the entire display region (image deleting), as shown in FIG. 13A.
In other words, all pixel electrodes 1 have the same electric potential (Vss), and a different voltage is applied to the common electrode 2 to have an electric potential (Vdd) (however, Vdd>Vss). As a result, an electric field (indicated by an arrow in FIG. 13A) from the common electrode 2 toward the pixel electrode 1 is generated between the pixel electrode 1 and the common electrode 2, the white particles 4 charged with the negative polarity move (migrate) toward the common electrode 2 by the electric field, and the black particles 3 charged with the positive polarity move (migrate) toward the pixel electrode 1. By driving in this manner, since the common electrode 2, functioning as the display surface, forms the background color by the white particles 4, the previous displayed image is deleted.
After that, new display image is rewritten on the display region (new image writing), as shown in FIG. 13B.
In other words, a voltage is selectively applied to the pixel electrodes 1a corresponding to display to make the electric potentials of the pixel electrodes changed to the electric potential (Vdd), and a different voltage is applied to the common electrode 2 to make the electric potential of the common electrode changed to the electric potential (Vss). As a result, a direction of the electric field is reversed only on the pixel electrodes 1a corresponding to display, so that the black particles 3 move toward the common electrode 2, and the white particles 4 move toward the pixel electrode 1a. On the other hand, in the pixel electrode 1b which is not corresponding to display and forms the background as it is, the common electrode 2 and the pixel electrode 1b become the same electric potential (Vss). Therefore, the particles 3 and 4 are held at locations at the time when deleting the image as they are, without the movement of the particles due to the removal of the electric field.
However, since the switching element or wiring line is generally connected to the pixel electrode 1 (1a and 1b), the pixel electrode is subjected to a voltage drop due to the channel resistance or wiring resistance and the influence of the wiring capacity or the like. As a result, the electric potential of the pixel electrode 1 (1a and 1b) becomes Vss′, not Vss, even though the voltage is applied thereto such that the pixel electrode has the Vss, as shown in FIGS. 14A and 14B. In other words, the Vss′ is a little larger than Vss.
If so, there is no problem when the image is deleted as shown in FIG. 14A. But, the electric potential difference between the electric potential (Vss) in the common electrode 2 and the electric potential (Vss′) in the pixel electrode 1b occurs in the pixel electrode 1b which forms the background when the new image is written as shown in FIG. 14B, so that a weak electric field from the pixel electrode 1 toward the common electrode 2 is generated. As a result, the particles 3 and 4 move a little from the locations at the time when deleting the image and a gray color is displayed at the portions on which the white color, functioning as the background color, must be originally displayed, thereby deteriorating contrast and image quality.